To set the rate of a mechanical watch, it is generally necessary to open the case and remove the movement, to then access the components for setting the rate: rotating the index to change the rigidity of the balance spring, rotating the balance screws to change the inertia, or other means. This operation therefore requires additional time-consuming operations. Moreover, it is also necessary to recheck the sealing. Sometimes, also, the rate may thrown out during the operation of casing up the movement.
In existing mechanisms, the movement must be disassembled to access the setting members, since the structure does not permit internal setting. Further, the risk of introducing unbalances during timing is not minimised.
CH Patent Application No 709052A2 in the name of Seiko instruments discloses a balance wheel composed of two parts, one of which is rigid and provided with two cams at 180°, and the other is composed of two resilient arms resting on the cams, which end in inertia blocks. A first rim forms the actual balance, and comprises a guide part configured to vary the distance, with respect to the balance staff, of a resilient part arranged to slide along the guide part, and which is capable of elastic deformation in the radial direction around the balance staff. A second rim comprises a plurality of inertia block portions. The relative rotation between these two parts causes a change in inertia through the radial travel of the inertia blocks. A variant is provided with a toothing allowing the insertion of a special tool ending in two pins; rotating this tool causes a precise tangential displacement of the inertia blocks. Although advantage is taken of the absence of play, this timing system requires disassembly of the movement in order for the tool to access the balance. This timing mode does not prevent the appearance of inadvertent unbalances during timing: the angular movement imparted by the tool at one of the ends risks producing a lower amplitude shift at the other diametrically opposite end, due to friction.
CH Patent Application No 708675A1 in the name of Sercalo Microtechnology Ltd describes a one-piece “LIGA” metal (Lithografie, Galvanoformung and Abformung) or “DRIE” (Deep Reactive Ion Etching) structure, comprising several elastic strips between an inner securing lozenge shaped part and a slightly elliptical outer ring, able to be secured by elastic forces inside a rim. Motion is started by rotating the outer resilient ring with the aid of tweezers, which moves the strips closer to or further from the centre, and changes the inertia. However, there is no integrated timing tool. Even using silicon technology, which can achieve very high manufacturing precision for this part, with the positioning of the elliptical ring being effected at two points, there is a risk of an unbalances appearing.
CH Patent Application No 320818A in the name of H. Siegwart also describes elastic strips and an elastic support resting inside the rim.